It is known that the ability of hot melt adhesives to be spread in their molten form is encumbered by rapid set up of the adhesive in the bond. It has been proposed to improve the spreading ability of a hot melt adhesive by forming a foam of the hot melt adhesive, applying it between substrates and pressing them together whilst the foam is still hot. U.S. Pat. No. 3,362,036 discloses extrusion of a mixture of hot melt adhesive and air or other inert gas by means of an extruder in which the adhesive is melted. UK Patent Specification No. 1562562 discloses forming a foam of hot melt adhesive and gas, pressurising the foam to force the gas into solution and then dispensing the adhesive solution. German Offenlegungsschrift No. 2935580 discloses the use of a blowing agent decomposable by heat to bring about foaming of a hot melt adhesive.
Proposals for providing hot melt adhesives in foam form have generally required use of equipment significantly different from that normally used for application of hot melt adhesives, or the formulation of adhesives with selected blowing agents.
Among objects of the present invention are to provide a method and means for providing a foamed mass of thermoplastic composition on a substrate using available compositions, especially hot melt adhesives, in conjunction with commercially available applying equipment appropriately modified by comparatively simple changes.